The long-term objective of the proposed research is to better understand human brain disorders that involve specific, learned, compulsive, behaviors. These include drug-taking in addiction, and (for example) repetitive hand-washing in obsessive compulsive disorder. These conditions have been linked to abnormal function of brain circuits that include the striatum. The proposed project will first examine normal learning processes in the striatum, that are thought to be responsible for the progressive automatization of frequently repeated behaviors. It is known that rats will initially perform certain maze tasks using a hippocampus-dependent spatial strategy, but then progressively switch to using a 'response' strategy that involves the striatum. During this transition, multiple electrodes will be used to record from neuronal ensembles in both hippocampus and striatum. The specific aim is to examine how the behavioral switch is reflected is altered neural representations of task components. In the second phase of the project, the effect of psychostimulant drugs on the alteration of these representations will be examined. Post-training injections of amphetamine have been shown to enhance learning of striatum-based behavioral strategies; the proposed project will address whether this facilitation electrophysiologically resembles normal learning.